The collecting duct of the mammalian kidney plays an important role in urinary acidification. Different portions of the collecting duct have been found to possess distinct acid-base transport properties. In this regard, the inner-stripe of the outer medulla has been identified as a segment where H is secreted into the tubule lumen by an active electrogenic process. Studies of this segment by other investigators have characterized the transepithelial transport properties of this segment, and found that mineralocorticoid hormone, cyclic-AMP, and alterations in systemic acid-base balance all act at this segment to regulate H secretion. Although much is known about the general features of transepithelial H secretion by this nephron segment, the details of the cellular and membrane mechanisms which underlie this process are largely unknown. Accordingly, the present proposal focuses on the examination of transport mechanisms and electrochemical gradients for H and HCO3 across the luminal and basolateral membranes of the H secreting cells. In addition, studies will be done to examine how mineralocorticoid hormone, cyclic-AMP, and alterations in systemic acid-base balance modulate these transport processes. The model system for these studies is the inner-stripe segment of the outer medullary collecting duct (OMCDi). Segments of OMCDi will be isolated from the kidney of the rabbit and perfused in vitro. Several different electrophysiological techniques will be used to study the process of H secretion at the cellular and membrane levels, including intracellular voltage recording microelectrodes, ion-selective microelectrodes, and patch clamp. It is expected that the results of these studies will provide new and important information on the mechanisms and regulation of urinary acidification. In particular, insight will be gained into the membrane transport processes associated with the electrogenic secretion of H by cells of the collecting duct. Such studies are also of general interest, since the viability of the cell and the organism as a whole is dependent upon the specific and directed movement of ions into and out of the cell. Thus, understanding the cellular and membrane mechanisms of epithelial (renal) ion transport, especially as related to whole- body electrolyte (acid-base) homeostasis is a problem of importance in both basic sciences and clinical medicine.